As a next-generation system of the W-CDMA (Wideband Code Division Multiple Access) and the HSDPA (High Speed Downlink Packet Access), an LTE system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of the W-CDMA. In the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied to be applied to the downlink communication system and the uplink communication system, respectively (see, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006, for example).
In the OFDM scheme, a frequency band is divided into plural sub-carriers having narrower frequency bands, and data are transmitted on each sub frequency band (sub-carrier) and the sub-carriers are closely arranged so as not to interfere with each other, so that fast data transmission can be achieved and an efficiency use of the frequency band can be improved.
In the SC-FDMA scheme, a frequency band is divided in a manner so that different frequencies can be separately used among plural terminals (user equipment terminals) and as a result, interference between terminals can be reduced. Further, in the SC-FDMA scheme, a range of transmission power fluctuation can be made smaller; therefore lower energy consumption of terminals can be achieved and a wider coverage area can be obtained.
The LTE system is a communication system using shared channels in both downlink and uplink. For example, in uplink, a base station apparatus selects a user equipment terminal to communicate using the shared channel in each sub-frame (each 1 ms) and instructs, using the downlink control channel, the selected user equipment terminal to communicate using the shared channel in a predetermined sub-frame. The user equipment terminal transmits the shared channel based on the downlink control channel. The base station apparatus receives and decodes the shared channel transmitted from the user equipment terminal. In this case, a process of selecting the user equipment terminal to communicate using the shared channel as described above is called a scheduling process.
Further, in the LTE system, so-called Adaptive Modulation and Coding (AMC) is applied; therefore, transmission formats of the shared channels may vary among different sub-frames. As used herein, the transmission format includes various information items indicating such as allocation information about resource blocks (frequency resources), a modulation scheme, a payload size, information about transmission power, HARQ (Hybrid Automatic Repeat reQuest) information (a Redundancy version parameter, a process number, etc.), and MIMO (Multiple Input Multiple Output) information (a reference signal sequence for MIMO transmission, etc.). The transmission format of the shared channel and identification information of the user equipment terminal which communicates using the shared channel in the corresponding sub-frame are collectively called an Uplink Scheduling Grant.
In the LTE system, the transmission format of the shared channel and identification information of the user equipment terminal which communicates using the shared channel in the corresponding sub-frame are reported using a Physical Downlink Control Channel (PDCCH). The Physical Downlink Control Channel (PDCCH) may also be called a DL L1/L2 Control Channel.